Symmetric Key Cryptography allows all parties must trust each other, because they can read each other's messages.

Physical distribution of symmetric keys is the safest because an unencrypted electronic transmission of a key would be vulnerable to theft. Physical key distribution would make symmetric keys hard to use in an environment with a large number of users. Even with Physical distribution, the keys are still vulnerable to theft.

However, often some application uses a clever combination of encryption schemes to distribute symmetric keys over the network for background authentication.